Fast charging of a battery is a major requirement for any high performance portable computer systems now. Many fast charging methods are currently being used. Out of these, pulse charging is a method in which a high current charge pulse is followed by a rest period and a discharge pulse before providing another driving charge pulse in sequence. This allows quick charging without impacting battery life. There are studies that show that battery capacity is reduced by 25% with conventional direct current (DC) charging compared to pulse charging when charged at the same rate after 750 cycles.
The discharge pulse in pulse charging helps proper distribution of charges within the cell and keeps the battery impedance at a minimum before the next high current charge pulse is applied. Without a preceding discharge pulse, the battery will operate at a higher voltage during charging, dissipate more power, and keep the battery at higher temperature. This can lead to dendrites outgrowth, metallic crystal formation and an increase in the internal resistance of the battery, which in turn leads to heat generation, poor battery charge efficiency, poorer battery capacity and shorter battery lifespan.
The size discharge pulses are usually approximately 2% of the duration of the charge pulse but more than twice the amplitude of the charge pulse. Recently released chargers that support fast charging use an external power dissipating resistor combined with a series switch to implement the discharge pulse. The power that needs to be dissipated by such a circuit can be 5% or more of the charging power itself. This puts considerable burden on the thermal solution that may be already operating at the boundary during fast charging. This can also limit the charging rate to a lower level due to the skin temperature and the battery temperature limitations due to the overall system losses and the losses in the battery itself.
Most of today's portable computing devices with a 1S battery use a step down type charger that takes input power from a standard Universal Serial Bus (USB) or USB Type-C connector. It also has a reverse boost feature to supply power back to the USB port from the battery in case a slave device is connected to the USB port. This is normally known as a reverse boost function. The same power path that drives the battery and system is used for the reverse power in boost mode. High conversion efficiency is possible due to the use of the same high power path.
High power computing devices such as Ultrabooks, 2-in-1s and workstations usually go with either a 2S, 3S or 4S battery. With USB-C getting the traction, there is a need for buck boost type chargers that can take input voltages from 4.5V to 21V and regulate the output voltage to the 2S-4S battery voltage range. The buck boost charger inherently supports the power flow in either direction irrespective of the voltage levels on either side.